PHYSICAL STABILITY OF STRUCTURED FLUIDS CONTAINING AIR BUBBLES

Home-care products are detergent systems that differ for applications, formulation and structure. From a physical point of view, detergents are usually suspensions where a surfactant-based, worm-like micellar solution represents the continuous phase and colloidal fibers are added in order to provide the matrices of specific properties, e.g. the ability to suspend pigments, oil droplets, perfumes. In spite of the many advantages assured by the presence of the fibers in terms of shelf life, the resulting system can be mechanically unstable. Basically, the main responsible for this instability is the load applied by air bubbles, which are found into the final product, due to the process itself. Academic and product-oriented researchers are interested in understanding the failure dynamics and, ultimately, in obtaining predictions on the physical stability of structured fluids over ageing. Hence, the aim of this work is to analyze the physical stability of a structured detergent in presence of air bubbles.We studied various samples, which differ from each other for aeration level and fiber concentration. The rheological behavior and the microstructure of these fluids has been characterized. In parallel, a time lapse photography technique has been used to monitor the time evolution of the air bubbles trapped in the fluids. The motion of single bubbles as well as the cooperative motion of bubble ensembles have been analyzed to verify the possibility of microstructure collapse. We found that fiber concentration, which dictates the yield stress of the fluid, aeration level and temperature can influence the stability of the final product. In particular, under given conditions, bubbles can apply a remarkable load on the fiber network during their buoyancy-driven rise, thus inducing the collapse of the structure. The result is a clear phase separation, with the matrix without fibers standing on the bottom of the fluid volume, while a more concentrated system is moving towards the free surface. Data have been collected, critically analyzed and compared with theoretical predictions and simulation model made by Comsol Multiphysics software